Excitation of Post-asymptotic Giant Branch Star Binary Eccentricity by Massive Polar-aligned Circumbinary Disks

Many post-asymptotic giant branch star binaries are observed to have relatively high orbital eccentricities (up to 0.6). Recently, AC Her was observed to have a polar-aligned circumbinary disk. We perform hydrodynamic simulations to explore the impact of a polar-aligned disk on the eccentricity of a binary. For a binary system with central masses of 0.73 and 1.4 M ⊙ , we find that a disk with a total mass of 0.1 M ⊙ can enhance the binary eccentricity from 0.2 to 0.7 within 5000 yr, or from 0.01 to 0.65 within 15,000 yr. Even if the disk mass is as low as 0.01 M ⊙ , the binary eccentricity grows within our simulation time while the system remains stable. These eccentricity variations are associated with the variations of the inclination between the disk and the binary orbit due to von Zeipel–Kozai–Lidov oscillations. The oscillations eventually damp and leave the binary eccentricity at a high value. The numerical results are in good agreement with analytical estimates. In addition, we examine the AC Her system and find that the disk mass should be on the order of 10 −3 M ⊙ for the disk to remain polar.